1. Field of the Invention
The present invention relates to a method for preparing poly(hydroxymethyl)-functional siloxanes and to poly(hydroxymethyl)-functional siloxanes.
2. Description of the Related Art
Poly(hydroxyalkyl)-functional siloxanes, including polysiloxanes, polysiloxane resins and organofunctional silica gels, incorporate as structural elements multiple units of the formula(siloxane-O)1+xSiRi2−x-Rii—OH,where Ri is an alkyl residue or an aryl residue, generally a methyl residue, Rii is a hydrocarbon residue, which may comprise or be substituted with heteroatoms, and which is attached to the silicon atom in the group SiRi2−x via a carbon atom, and x=0, 1 or 2.
Siloxanes, polysiloxanes, polysiloxane resins and organofunctional silica gels are together referred to below as “siloxanes”. “Heteroatom” is understood to mean every atom except carbon and hydrogen, in particular nitrogen, oxygen, halogen, silicon, phosphorus and sulfur.
The presence of Rii between the silicon atom and the depicted OH group has the effect that the bond attaching the OH group to the siloxane skeleton is stable to hydrolysis. If the OH group is reacted with other compounds, e.g. in polyaddition reactions with, for example, isocyanates or in polycondensation reactions with, for example, carboxylic acids, the bond attaching the resulting products to the siloxane skeleton will likewise be stable to hydrolysis.
The group Rii is in this case a structure-conferring factor, which co-determines not only the properties of the poly(hydroxyalkyl)-functional siloxane but also the properties of the conversion products. It is especially both the mobility of Rii and the organic character of Rii which influence these properties (e.g. hardness, flammability or hydrophilicity). If, for example, the mobility of Rii and/or the organic character of a poly(hydroxyalkyl)-functional siloxane or the conversion products thereof are to be kept to a minimum, the smallest possible Rii residues are ideal, and the choice of Rii as CH2 is particularly advantageous. A further advantage of this choice for Rii is that small structural units mean lower reaction volumes for the same amount of substance with Rii-attached OH groups and hence enhanced space-time yields both in the preparation of poly(hydroxyalkyl)-functional siloxanes and conversion products thereof. The CH2 group is, in this respect, the most efficient solution.
Moreover, the CH2 group is likewise the most advantageous solution when the hydrophilicity of the poly(hydroxyalkyl)-functional siloxane is to be as high as possible. Compared to Si—OH groups, the Si—CH2—OH unit has the advantage that condensation reactions with one another, which alter the Si—O skeleton of the poly(hydroxyalkyl)-functional siloxane, cannot occur. The degree of hydrophilicity can be controlled by the number of hydroxyalkyl residues and can range up to water-solubility. In this manner, siloxanes can be accorded a property which can otherwise only be achieved with difficulty in this substance class.
Poly(hydroxyalkyl)-functional siloxanes, where Rii is equal to CH2, are referred to in the poly(hydroxymethyl)-functional siloxanes below.
Methods for preparing poly(hydroxyalkyl)-functional siloxanes are documented in the literature. U.S. Pat. No. 3,879,433, for example, describes the preparation of poly(hydroxyalkyl)-functional siloxanes by hydrosilylation of hydroxyolefins. However, only hydroxyalkyl groups having at least three carbon atoms are accessible in this manner.
The preparation of poly(hydroxymethyl)-functional siloxanes is possible by acid-catalyzed alcoholysis of corresponding poly(acyloxy)methyl-functional siloxanes (e.g. DE 879839) or (acyloxy)methylsilanes (e.g. DE 1236505), in the latter case combined with cohydrolysis and co-condensation with further silanes.
Additional methods are described for the introduction of terminal or single lateral hydroxymethyl groups into siloxane molecules:                DE 879839: acid- or base-catalyzed equilibration of 1,3-bis(acyloxymethyl)tetramethyldisiloxane with further silanes with simultaneous alcoholytic ester cleavage,        U.S. Pat. No. 2,837,550/J. Am. Chem. Soc. 77 (1955) 5180: Grignardization of chloromethylsiloxanes and subsequent oxidation and hydrolysis,        DE 1213406: hydroxylation of bromomethylsiloxanes with metal hydroxides,        DE 1227456: acid-catalyzed equilibration of 1,3-bis(hydroxymethyl)tetramethyldisiloxane with further silanes,        DE 1233395: reductive cleavage of (acyloxy)methylsiloxanes with boranates in the presence of boron trifluoride,        DE 102009046254: reaction of terminally OH-functionalized polysiloxanes with cyclic or acyclic alkoxysilanes.        
The methods described to date for preparing (hydroxymethyl)-functional siloxanes have multiple disadvantages:
1. Under the reaction conditions described, particularly in the equilibration methods, slight rearrangements of the siloxane skeleton occur such that the methods do not result in defined products (DE 879839, DE 1236505, DE 1213406, DE 1227456, DE 1233395).
2. Grignard compounds are expensive due to the magnesium metal required. The oxidation reaction is strongly exothermic and therefore difficult to manage on an industrial scale and not without hazard. Hydroxymethyl groups, moreover, are only formed in low yield (U.S. Pat. No. 2,837,550).
3. The liberation of the ≡SiCH2OH groups from the corresponding precursor compounds (e.g. ≡SiCH2—Oacyl or ≡SiCH2-halogen) frequently does not proceed quantitatively (DE 879839, U.S. Pat. No. 2,837,550, DE 1236505, DE 1213406, DE 1227456, DE 1233395).
4. Boronates are costly and, also like boron trifluoride, hazardous reagents (DE 1233395).
5. The ≡SiCH2OH groups formed react further under the reaction conditions (e.g. with HCl to give ≡SiCH2Cl groups, with acidic catalysts, e.g. sulfuric acid, to give ≡SiCH2OCH2Si≡ groups or with hydroxides by cleavage of the Si—C bond of the SiCH2OH groups to give Si—OH groups) such that the product does not have the theoretically expected number and concentration of ≡SiCH2OH groups (DE 879839, DE 1236505, DE 1213406, DE 1227456, DE 1233395).
6. Reagent residues and/or catalyst residues in the product frequently lead to rearrangement, cleavage, condensation or equilibration of the siloxane skeleton, such that the properties of hydroxymethyl-functional siloxanes, which were prepared by the methods described to date, frequently change on storage (DE 879839, DE 1236505, DE 1213406, DE 1227456, DE 1233395).
7. The methods provide only terminal, and therefore a limited number of, hydroxymethyl groups (DE 1227456, DE 102009046254).
All this complicates or prevents the preparation of poly(hydroxymethyl)-functional siloxanes and the further processing thereof to give defined conversion products and this applies especially to subsequent reactions of the SiCH2OH group.
The preparation of poly(hydroxymethyl)-functional silica gels by hydrolysis of hydroxymethyltrialkoxysilanes has been described:                JP 63048364: cohydrolysis with further silanes for surface coating,        DE 4407437: sol-gel hydrolysis for preparing thin layers,        U.S. Pat. No. 6,310,110: emulsion hydrolysis in the presence of surface-active substances having template groups for preparing micro- or nanostructured porous particles,        US 2007 0269662: sol-gel hydrolysis for preparing thin membranes,        US 2008 0268162: hydrolysis, optionally in the presence of surface-active substances, for surface coating,        J. Coll. Interface Sc. 340 (2009) 202-208: sol-gel cohydrolysis with a further silane in the presence of a surface-active substance for preparing particles with corn-shell structure,        DE 10044635: cohydrolysis for encapsulating active ingredients.        
A common aspect of the methods referred to is that they use an alcoholic solution of a hydroxymethyltrialkoxysilane as starting material. This also comprises condensation products having Si—CH2—O—Si units in the equilibrium, specifically more so the higher the silane concentration (DE 4407437). This is disadvantageous since complete hydrolysis is no longer ensured at a high degree of condensation, since not all Si—OR or Si—CH2—O—Si units are accessible to hydrolysis, due to the resulting strong cross-linking and steric shielding. If a high alcohol dilution is employed in order to avoid this issue, this can likewise be disadvantageous since a free choice of silane concentration and solvent is no longer possible. However, these are important parameters for controlling the size and morphology of the resulting silica gel particles.
All this complicates or prevents the preparation of poly(hydroxymethyl)-functional silica gels, particularly having defined structure and morphology, and the further processing thereof to give defined conversion products and this applies especially to subsequent reactions of the SiCH2OH group.